Frequency converter



6 Sheets-Sheet l Qxiginal Filed July 17 1961 ivm .FDAPDO ATTORNEY Aug.l2, 1969 P. o. coREY FREQUENCY CONVERTER Original Filed July 1'7. 1961Nam.

.m v @E mw Y/ .mzp 55m z3 5F52: mwEmmL 6 Sheets-Shea*l 3 Original FiledJuly 1'?. 1961 w35? Y n Dm n L M m o o @ma m U D A Mw mm PEE wm L u EzxJp @E zojwo N wmrur i Il Y B 6 Sheets-Sheet -1 Original Filed July 1'?,1961 Nmm mmm

we. vl mmm a m m m O R n m m V A W P w n. C H P wm M a* 2P 250m 2350zmmm i2 Saz. ,EzOU 62.565 SQ 55E mmwmw SZWEEE 99E M NSM co1 Saz. U

Aug. l12, 1969 P. o. coREY FREQUENCY CONVERTER 6 Sheets-Sheet L OriginalFiled July 17 1961 wzutxw Y IIL m" 'FNTUR PHQLIP UCOREY m' g Z ATTORNEYAug. l2, 1969 P. n. COREY Re- 26541 FREQUENCY CONVERTER Original FiledJuly 1'7, 1961 6 Sheets-Sheet f' PHILIP D, COREY ATTORNEY United StatesPatent O 26,641 FREQUENCY CONVERTER Philip D. Corey, Crozet, Va.,assignor to General Electric Company, a corporation of New York OriginalNo. 3,248,635, dated Apr. 26, 1966, Ser. No.

124,467, July 17, 1961. Application for reissue Apr. 22,

1968, Ser. No. 741,821

Int. Cl. H02m 3/22, 5/40 U.S. Cl. 321-4 37 Claims Matter enclosed inheavy brackets appears iu the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade hy reissue.

ABSTRACT F THE DISCLOSURE Apparatus for converting power of onefrequency to a different frequency includes a rectifier circuit changingan input into D.C. power, a regulated, D.C` supply voltage source, anoscillator' circuit operated at a stable freo quency by the voltagesource, and an inverter circuit having as an input the D.C. power, theinverter circuit being diven by the oscillator circuit to produce apower output of the desired frequency. Voltage regulation of the poweroutput may be accomplished by providing a master and a slave oscillatorin the oscillator circuit, an output of the master oscillator beingcoupled to the slave oscillator through a magnetic phase shifter, and byproviding corresponding inverters in the inverter circuit whose outputsare vectorially combined. By comparing the power output voltage with avoltage reference, any dierence therebetween is applied as a correctionvoltage to control the magnetic phase shifter so that the phasedisplacement between master and slave oscillations and the correspondinginverters can be varied. Frequency regulation may be accomplished bycomparison of the D.C. supply voltage with a frequency reference voltageand control thereby of a magnetic amplifier within the voltage source.Various embodiments of the oscillators including magnetic coupledmultivibrators are illustrated, as well as embodiments of the magneticphase shifter including various magnetic ampliers and a saturablereactor.

This invention relates to frequency converters. More particularly, it-relates to a system for converting a [polyphase] power input of onefrequency to a single phase power output of a different frequency.

At present, the systems that are utilized for frequency conversion of[polyphase] power inputs of one frequency to a single phase power outputof a different frequency essentially employ rotary equipment containingmechanical parts. Such rotary equipment is expensive, quite bulky andheavy, requires constant maintenance, and does not afford a sufficientdegree of reliability.

Accordingly, it is an important object of this invention to provide asystem for converting a [polyphase] power input thereto of one frequencyto a single phase power output of a different frequency, such systembeing static, i.e., containing substantially no moving parts.

It is a further object to provide a system in accordance with thepreceding object which is of much lighter weight than heretofore knownsimilar type equipments, and which is highly reliable.

It is another object of the invention to provide a frequency converterin accordance with the preceding objects which has a high tolerance toinput voltage transients, has highly reliable operation at a wide rangeof operating temperatures, and is relatively simple in circuitarrangement.

Generally speaking and in accordance with the invenlCe tion there isprovided a combination adapted to be coupled to an alternating currentpotential source [having] which may have a plurality of balanced phaseoutputs, each of the outputs having a voltage and a frequency which mybe randomly variable comprising means adapted to be connected in circuitwith the [polyphase] source for rectifying its outputs, an oscillatorhaving a chosen frequency, and a power inverter. There are furtherincluded means for applying the output of the oscillator and the outputof the rectifying means to the power inverter to produce a single phasepower output having the oscillator frequency. A source of referencepotential of a chosen value is provided and such reference potential iscompared with the output voltage from the power inverter, the voltageresulting from such comparison being applied as a correction voltage t0the power inve-rter to regulate the voltage of the system output.

A feature of the invention resides in the use of silicon controlledrectifiers as the switching elements in the power inverter.

Another feature of the invention resides in the rectifying of the[polyphase] source input, inverting the unidirectional potentialproduced by such -rectifying to an A.C. output, and regulating thevoltage of the A.C. output. The oscillator in the system is suitablychosen to be of the square wave output type whereby the output of theinverter is a quasi square wave.

In one illustrative embodiment of the invention, a feature resides inthe use of a master square wave oscillator and a slave square waveoscillator having identical outputs but which are displaced in phase inaccordance with the magnitude of the correction voltage. The powerinverter of this embodiment also comprises separate respective inverterswhose outputs are controlled by the master and slave oscillatorsrespectively whereby their outputs are also displaced in phase inaccordance with the magnitude of the correction voltage. The outputs ofthe inverters comprising the power inverter are combined by phasoraddition, and such output voltage is then compared with the referencevoltage.

In another illustrative embodiment of the invention, a features residesin directly varying the conduction intervals of the switching devices inthe power inverter t0 provide voltage regulation of the output of thesystem.

The novel features, which are believed to be characteristic of thisinvention, are set forth with particularity in the appended claims. Theinvention itself, however, both as to organization and method ofoperation together with further objects and advantages thereof, may bestbe understood by reference to the following description when taken inconnection with the accompanying drawings.

In the drawings, FIG. 1 is a block diagram of an embodiment of afrequency converter according to the invention;

FIGS. 2 and 3 taken together as in FIG. 4 is a schematic depiction ofthe frequency converter shown in block form in FIG. 1;

FIG. 5 is a block diagram of another embodiment of a frequency converteraccording to the invention;

FIG. 6 is a schematic diagram of another embodiment of the master-slaveoscillator combination shown in FIG. l and FIGS. 2 4;

FIG. 7 is a diagram partly in block form and partly schematic of anembodiment of a master-slave oscillator combination utilized to regulatethe output voltage resulting from the combining of the outputs of aplurality of power inverters; and

FIG. 8 is a graph which shows the advantageous smoothness of phase shiftprovided by such muster-slave oscillator combination, the data for suchgraph being ob- 3 tained from the operation of the master-slaveoscillator combination in the circuit of FIGS. 2-4.

Referring now to FIG. l, [a multiple balanced phase] an input shown forconvenience as comprising three phases and which may have a randomlyvariable voltage and frequency is passed through a high power rectifierl to provide a full wave combined rectified output of the polyphaseinput. Rectifier may suitably be a three phase "double way" rectifier.The output of rectifier 10 is passed through a D.C. smoothing filter 12to smooth the output of rectifier [12] [O and thereby to eliminatevoltage modulation of the output of the system. Filter 12 may suitablybe of the well known LC choke filter type, the LC ratio therein beingchosen to be of a small value to minimize voltage transients due to stepchanges in the load of the output of the system.

The smooth but unregulated output from filter 12 which is aunidirectional potential, [suitably of about 250 volts,] is applied tothe power inverter stage 14. Stage 14 comprises two identical bridgeinverters 16 and 18 which produce square wave power outputs of a chosenfrequency in response to the concurrent application thereto of theunidirectional input from filter 12 and a square wave voltage havingsuch chosen frequency. 'Ihe inverters 16 and 18 contain power switchingdevices which are suitably silicon controlled rectifiers. Inverters 16and 18 may include output transformers, the secondary windings of whichare connected in series so that the total output of the inverter stageI4 is the phaser sum ofthe outptlts on the windings which are `soconnected in series. Stich phasor addition is conceptually depicted asbeing effected in combining stage 20.

The total output of power inverter stage 14 is filtered in a filterstage 22. filter stage 22 preferably being a so-called "fourth order"type filter. The latter type filter may comprise a series resonantinductor and capacitor in series arrangement with the output of thepower inverter stage 14 and a parallel resonant inductor and capacitorconnected directly across the terminals of power inverter stage 14. theinductor-capacitor circuits being tuned to the frequency of theoscillators. The inductor in the series resonant portion of filter stage22 may be chosen to be of saturable type and having a volt-secondcharacteristic wherein it saturates at overload current. The use of suchsaturable series connected inductor serves to automatically increase theimpedance of the series connected resonant circuit in hlter stage 22 andthereby limits the output current of the system to a safe value. Filterstage 22 is preferably designed so that over a prescribed load and powerfactor range. the total harmonic content in the output of the systemdoes not exceed a small percentage such as about 5 percent.

The square wave voltage inputs to bridge inverters 16 and 18respectively which determine the frequency of the output of the systemare produced in master oscillator 24 and slave oscillator 26.Oscillator-s 24 and 26 conveniently may be magnetic coupledmultivibrators whose frequencies are functions respectively of the D.C.supply voltage applied thereto. Such supply voltage is preferablyclosely regulated to maintain the output frequencies of oscillators 24and 26 within close tolerances.

To provide a reglated voltage supply for oscillators 24 and 26, aportion of the polyphase inptlt to the system is passed through a lowpower transformer 28. The outputs of transformer 28 are applied to thegate windings of a relatively high gain self-saturating magneticamplifier 30, i.e., an "amplistat," A control winding in magneticamplifier 30 has applied thereto [to] a D.C. voltage such that the valueof the output voltage of magnetic amplifier 30 is the desired regulatedDC. supply voltage for oscillators 24 and 26.

In this connection. the voltage supply for oscillators 24 and 26 iscompared with the output of a reference voltage source 32, theresuitably being developed in stage 32, the proper voltage across areference diode such as a Zener diode. The supply voltage foroscillators 24 and 26 and the voltage from source 32 are compared. suchcomparison being depicted conceptually as being effected in element 34and the difference therebetween is the aforesaid D.C. voltage which isapplied [as the correction voltage] to the control winding of magneticamplifier' 30. The [correction] D.C. voltage is applied to the controlwinding in such polarity whereby the output of magnetic amplifier iseither increased or decreased as is necessary to maintain the supplyvoltage for oscillators 24 and 26 at the desired regulated level. [TheD.C. input to oscillators 24 and 26 may suitably be about l5 volts] Theoutput of master oscillator Z4 is applied as a driving input to bridgeinverter 16 and is a square wave voltage which determines the outputfrequency of bridge inverter 16. The output of master oscillator 24 isalso applied as an inptlt to slave oscillator 26 through a magneticphase shifter stage 36. Slave oscillator 26` is suitably a circuitsimilar to master oscillator 24 and magnetic phase shifter 36 may be acoupling between oscillators 24 and 26 such as a saturable inductor or amagnetic amplifier which has a chosen volt-second characteristie wherebythe output of slave oscillator 26 is displaced in phase with respect tothe output of master oscillator 24 an adjustable amount, such amountbeing in accordance with the value of the voltage applied to magneticphase shifter 36 and its volt-second characteristic. The output fromslave oscillator 26 drives bridge inverter 18 whereby the output ofbridge inverter 18 lags the output of bridge inverter 16 the same amountas the output of the slave oscillator 26 lags the output ol masteroscillator 24.

In the system of FIG. l, the output voltage may be sensed by rectifyingthe output of lter 22 to obtain a unidirectional voltage whose value isproportional to the average of the A.C. voltage output from filter 22.Such unidirectional voltage is then compared with the voltage from avoltage reference source 38, the reference voltage being the propervalue to provide the desired system output voltage. Such referencevoltage from source 38 is suitably developed across a reference diodesuch as a Zener diode. The comparison of the unidirectional voltage andthe reference voltage from source 38, conceptually depicted as beingeffected in element 40 provides a difference or correction voltage whichis applied to magnetic phase shifter 36, i.e., to the control winding ofa magnetic amplier or to a saturable inductor. Hence, changes in thecontrol current in magnetic phase shifter 36 effect rapid and accuratecontrol of the phase displacement between the output of masteroscillator 24 and slave oscillator 26 and consequently, between theoutputs of bridge inverters 16 and 13. Where magnetic phase shifter 36is chosen to be a magnetic amplifier, there may be included a separatecontrol winding in the magnetic amplifier which is shunted by a resistorand inductor to achieve lag-lead compensation of the frequency responsecharacteristic of the magnetic amplifier. Such resistor and inductor areso designed as to optimize transient response of the total system ofFIG. l to input line voltage fluctuations as well as abrupt output loadchanges.

Referring now to FIGS. 2-4, the [polyphase] input power supply to thesystem, again for convenience of explanation is shown to have threephases equally displaced in phase with respect to each other. Thefrequency and voltage of the input may be randomly variable and theremay be any number of phases.

The polyphase inputs are passed through low pass filters 42, 44 and 46.Filter 42 comprises a series connected inductor 48 and parallelconnected capacitors 50 and 52, filter 44 comprises a series connectedinductor 54 and parallel connected capacitors 56 and S8 and filter 46comprises a series connected inductor.6t) and parallel connectedcapacitarse 62 and 64.

The outputs of filters 42, 44 and 46 are applied to the three phase fullwave high power rectifier 10. Rectifier comprises a rst portion 66 whichcomprises a series arrangement of diodes 68 and 70 in shunt with aseries arrangement of a resistor 72, a capacitor 74, a resistor 76 and acapacitor 78, the junction 75 of capacitor 74 and resistor 76 beingconnected to the junction 69 of the cathode of diode `68 and the anodeof diode 70.

A second portion 80 of rectifier 10 comprises the series connecteddiodes 82 and 84 in shunt with the series arrangement of a resistor 86,a capacitor 88, a resistor 90 and a capacitor 92, the junction 89 ofcapacitor 88 and resistor 90 being connected to the junction 83 of thecathode of diode 82 and the anode of diode 84,

A third portion 94 of rectifier 10 comprises the series connected diodes96 and 98 in shunt with the series arrangement of a resistor 100, acapacitor 102, a resistor 104, and a capacitor 106, the junction 103 ofcapacitor 102 and resistor 104 being connected to the junction 97 of thecathode of diode 96 and the anode of diode 98.

The output of rectifier 10 is passed through smoothing filter 12comprising a series connected choke coil 108 and a parallel connectedcapacitor 110 to provide at junction 109, a relatively smooth,unregulated, unidirectional potential. The L to C ratio of choke 108 andcapacitor 110 is chosen to be relatively small to minimize voltagetransients due to the step changes in the load of the output of thesystem.

A portion of the A.C. input to rectifier 10 is applied to primarywinding 114 of low power transformer 28, primary winding 114 beingconnected between junctions 89 and 97.

[The] Referring now to FIG. 3, the voltage appearing at the midpoint ofsecondary winding 116 is developed across a resistor 118 and then passedthrough a filter comprising a series connected choke 120 and a parallelconnected capacitor 122.

The filtered voltage appearing at the junction 121 of choke 120 and acapacitor 122 is developed across the series arrangement of a resistor124 and the cathode to anode path of a reference diode 126 (Zener, forexample) and is also developed across a parallel connected variableresistor 128. The value of resistor 124 is so chosen [whereby thevoltage across reference diode 126 has the desired value for the outputsupply voltage to master and slave oscillators 24 and 26] to result inoptimum current so that a sutlstantially constant D.C. voltage isdeveloped acrOss diode 126.

Connected between the junction 125 of resistor 124 and Zener diode 126and a point 129 on resistor 128 is a control winding 130 of magneticamplifier 30. Magnetic amplifier 30 comprises one gate winding 132connected to terminal 115 of secondary winding 116 in FIG. 2 andconnected in series with the cathode to anode path of a diode 134 andanother gate winding 136 connected to terminal 112 of secondary winding116 and connected in series with the cathode to anode path of a diode138, diodes 134 and 138 serving to provide amplistat gain in magneticamplifier 30 and at the same time to provide a D.C. supply voltage foroscillators 24 and 26.

The polarity [dot] mark designations on control winding 130 and gatewindings 132 and 136 of magnetic amplifier 30 indicate the direction ofcurrent fiow therethrough to produce positive ampere turns therein.Accordingly, it is seen that in the event that the voltage at junction125 exceeds the voltage at point 129 on resistor 128, the direction ofcurrent through control winding 130 is such as to increase the output ofmagnetic amplifier 30 whereby the average voltage developed acrossresistor 118 is increased and in the event that voltage at point 129exceeds the voltage at junction 125, the direction ot' current throughcontrol winding 130 is such as to decrease the output of magneticamplifier 30 whereby the average voltage developed across resistor 118is decreased. lsolated control winding 140 in series arrangement withvariable resistor 142 is a second control winding for magnetic amplifier30. Control winding functions to slow the operation of magneticamplifier 30 and to filter the voltage sensed on control winding 130whereby there is well-damped voltage regulation in response totransients. It is accordingly seen that magnetic amplifier 30 functionsto provide a regulated D.C. supply voltage for oscillators 24 and 26.Diode 150 functions to negatively clamp the voltage appearing at point152 to the voltage appearing at point 146, and diode 144 functions todecouple the voltage across diode from magnetic amplifier 30.

As will be further explained hereinbelow, master oscillator 24 doescomprise and slave oscillator 26 may comprise a saturableautotransformer. [The] A saturable autotransformer 154 for masteroscillator 24, for example, comprises two identical cores. A winding 156thereof encompases one of the cores and a winding 158 encompases theother of the cores. The twin cores of saturable transformer 154 aretaped together respectively with the windings 156 and 158 thereon asdescribed. The other windings of saturable transformer 154, i.e., theprimary and secondary windings thereof are wound around the tapedcombination.

An operating coil K of a relay is connected across the regulated D.C.supply voltage from point 121 to point 146. Normally-closed contacts K1thereof connect point 121 through the series connection of windings 156.158, a control winding 249 of a magnetic amplifier 230 hereinafter to bedescribed, a resistor 166 and a capacitor 170 to a common point 164, thejunction between resistor 166 and capacitor 170 being connected toground. Similarly, normally-open contacts K2 connect point 121 through adiode 168 to point 164` Considering the operation of windings 156 and158, when the regulated D.C. voltage appearing at point 121 is passedthrough the operating coil of relay K and simultaneously passed throughnormally closed contacts Kl associated therewith, due to the polaritiesof windings 156 and 158 respectively, as shown by the designatingpolarity [dots] marks thereon, the current fiowing in the same directionthrough windings 156 and 158, control winding 249 of a magneticamplifier 230 and resistor 166 orients the core material of the twocores of transformer 154 in opposite directions. Such oppositeorientation effectively presets oscillator 24 to an initial condition aswill be further explained hereinbelow.

After the operating coil of relay K is energized, normally closedcontacts K1, [associated therewith] assume the open position andnormally open contacts K2 [associated therewith] assume the closedposition whereby the regulated D.C. voltage supply from point 121 can beapplied to master and slave oscillators 24 and 26 through diode 168, [afilter] capacitor 170 [being provided from point 164 to the negativeterminal of the D.C. supply] Serving as a filter.

Master oscillator 24 comprises a first transistor 172 having an emitter174 directly connected to the positive terminal (i.e., point 164) of theregulated D.C. voltage supply, a collector 176 connected to the negativeterminal 146 of the regulated D.C. supply through a primary winding 180of transformer 154, the emitter 174 being connected to the junction 181of the negative terminal 146 of the D.C. supply and [junction 181]winding 180 through the series arrangement of resistors 177 and 178, anda base 176 connected to the junction 179 of resistors 177 and 178through a secondary winding 182 of transformer 154.

A second transistor in oscillator 24 has its emitter 192 connected toemitter 174, its collector 194 connected to junction 181 through aprimary winding 186 of transformer 154 and a base 196 connected tojunction 179 through a secondary winding 184 of transformer 154. As hasbeen aforestated, transformer 154 is of the saturable type and maysuitably be an autotransformer, the

core material therein preferably being of a grain oriented magneticmaterial having a given volt-second characteristic, i.e., the product ofthe voltage applied thereto and the time required for the cores thereofto go from saturation in one direction to saturation in the oppositedirection.

Slave oscillator 26 is essentially similar to master oscillator 24 andaccordingly, is also a magnetic coupled square wave multivibrator.However, the transformer 162 in slave oscillator 24 need not be ofsaturable type. If it is of the saturable type, then the volt-secondcharacteristic of its core material has to be greater than that oftransformer 154 as will be further explained.

ln slave oscillator 26, a first transistor 200 has its emitter 202connected to positive terminal 164 of the regulated D.C. Supply. itscollector 204 connected to negative terminal 146 of the D.C. supplythrough a primary winding 208 of transformer 162, emitter 202 beingconnected to the junction 209 of negative terminal 146 of the D.C.supply and primary winding 208 through the series arrangement ofresistors 216 and 218, and a base 206 connected to the junction 217 ofresistors 216 and 218, through a secondary winding 212 of transformer162.

A Second transistor 220 in slave oscillator 26 has its emitter 222connected to emitter 202, its base 226 connected to junction 217 througha secondary winding 214 of transformer 162 and its collector 224connected 'lo junction 209 through a primary winding 210 of transformer162.

A twin cored magnetic amplifier 230 which is an embodiment of themagnetic phase shifter 36 of FIG. 1 comprises gate windings 232 and 234having their respective terminals 233 and 237 connected together, thejunction 236 of windings 232 and 234 being connected to base 226 oftransistor 220, the other terminals 231 and 235 respectively of gatewindings 212 and 234 having connected therebetween the anode to cathodepaths of diodes 238 and 240. The non-polarity [dot] mark terminal of asecondary winding 185 of transformer 154 is connected to junction 239 ofthe cathode of diode 238 and the anode of diode 240 and the polarity[dot] mark terminal of secondary winding 185 is connected to base 206 oftransistor 200. A control winding 242 of magnetic amplifier 230 isconnected in the [output voltage sensing circuit,] element 40, therebeing developed thereacross an error or correction voltage which resultsfrom the comparison between the output voltage of the system and areference voltage of a desired value. A [Control] winding 244 ofmagnetic amplifier 154 in series arrangement with a resistor 246 is anisolated control winding which has the dual function of slowing theoperation of magnetic arnplitier 230 and filtering the voltage sensed oncontrol winding 242 whereby there is provided a well damped voltageregular response to transients, the operation of winding 244 beingsimilar to the operation of winding 140 in magnetic amplifier 30.

Considering the operation of master oscillator 24 and slave oscillator26 in conjunction with magnetic amplifier 230 including control winding242, normally in the operation of a multivibrator such as thatcomprising transistors 172 and 190 and saturable transformer 154,transistors 172 and 190 alternately apply the voltage from the D.C.supply, i.e., from points 164 and 146, to primary windings 180 and 186of transformer 154. Upon the application of such voltage, the voltagedivider comprising resistors 177 and 178 biases the base to emitterjunctions of both transistors 172 and 190 in such a direction as torender them both conductive. However, any small unbalance causes onetransistor to become conductive before the other. lf it is assumed thattransistor 172 is rendered conductive first, the polarity of winding 182is such that when transistor 172 conducts, the positive voltage appliedat the nonpolarity [dot] mark terminal of winding 182 induces a negativevoltage at base 176 with respect to the junction 179, thereby increasingthe conductivity in transistor 172 and holding it conductive untiltransformer 154 saturates a constant number of volt-seconds later. Whiletransistor 172 is so biased in the conductive direction, it is to benoted that the reverse polarity occurring in winding 184 is biasingtransistor 190 further in the nonconductive direction. When transformer154 saturates after transistor 172 has been conductive, the base driveon transistor 172 collapses and transistor 190 is substantiallyimmediately rendered conductive. In this manner, transistor 190 suppliesthe other half of the output cycle of the multivibrator.

In the event that transformer 162 is a saturable transformer, themultivibrator comprising transistors 200 and 220 by itself operates inthe same manner as described in connection with the multivibratorcomprising transistors 172 and 190'. The volt-second characteristic oftransformer 162 in the event that it is chosen to be of the saturabletype, has to be greater than the volt-second characteristic transformer154, whereby the natural frequency of slave oscillator 26 is less thanthat of master oscillator 24.

Now considering the operation of both oscillators 24 and 26 and themagnetic phase shifter 30 or magnetic amplifier 230 couplingtherebetween, it is seen that outputs of transistors 172 and 190 ofmaster oscillator 24 are applied to gate windings 232 and 234respectively of magnetic amplifier 230I through secondary winding 185.The [control] correction voltage derived from the comparison between thesystem output voltage and the reference voltage is generated on controlwinding 242. The polarity [dots] marks on the windings of magneticampliiier 230 indicate the direction of current therethrough to producepositive ampere turns therein and thereby increase the output of themagnetic amplifier.

lf it is assumed that transistor 172 of master oscillator 24 andtransistor 220 of slave oscillator 26 are concurrently conducting, it isseen that `current from the nonpolarity [dot] mark terminal of secondarywinding is passed through diode 240 and through gate winding 232 to base[206] 226 of transistor 220. Dependent upon the volt-secondcharacteristic of the core material of magnetic amplifier 230, whenmagnetic amplifier 230 saturates due to the [current through] voltageapplied to gate winding 232, the sudden drop in the impedance of winding232 and the consequent rise in potential at base 226 rapidly renderstransistor 220 nonconductive and by transformer action, transistor 200is consequently rapidly rendered conductive.

It has been stated above that transformer 154 is of the saturable typebut that transformer 162 may be of the unsaturable type. If transformer162 is chosen to be of the saturable type, it has to have [an NABsproduct] a volt-second characteristic which is appreciably greater thanthe [NABs product] volt-second characteristic of transformer 154, [thedifference being about 25 percent] The natural frequency of slaveoscillator 26 is consequently appreciably less than that of masteroscillator 24. The volt-second characteristic of the core material ofmagnetic amplifier 230 and the error or correction voltage generated oncontrol winding 242 determines the amount of phase displacement betweenthe outputs of oscillator 24 and oscillator 26.

It is to be further noted that core material of magnetic amplifier 230has to be chosen to have a volt-second characteristic whereby its timeof switching from saturation in one direction to saturation in the otherdirection cannot eitceed the time of a half cycle of output fromoscillator 24. If its volt-second characteristic were so chosen wherebyits saturation time could be longer than the period of such half cycle,then [in the event. of course, that transformer 162 were chosen to be ofthe saturable type, the frequency of the output of oscillator 26 wouldbe its natural frequency as determined by the volt-second characteristicof transformer 162 and the value of the regulated D.C. supply voltage.ln this type situation, oscillator 24 could not control the outputfrequency of oscillator 26] oscillator 26 would remi to mm at itsnatural frequency, as determined by the inherent volt-secondcharacteristic of transformer 162. Loss of synchronized, controlledphase shift operation between oscillators 24 and 26 would result as wellas erratic operation of inverters 16 and 18.

Accordingly, with the arrangement of master oscillator 24, slaveoscillator 26 and the magnetic amplifier 230 coupling therebetween, thephase dilference permitted between the outputs of oscillator 24 andoscillator 26 is up to a maximum of 180. It is, of course, appreciatedthat if volt-second characteristic of transformer 162, in the event thatit were chosen to be of the saturable type, were equal to or less thanthe volt-second characteristic of transformer 154, oscillator 26 wouldhave a natural output frequency independent of the frequency ofoscillator 24. If magnetic amplifier 230 were eliminated from thecircuit, and transformer 162 were either of the nonsaturable type or ofthe saturable type and having a greater voltsecond characteristic thanthat of transformer 154, the output of oscillator 26 would be insynchronism with the output of oscillator 24 with no phase differencebetween the outputs. Diodes 238 and 240 elect high amplistat gain inmagetic amplifier 230.

The arrangement comprising oscillators 24 and 26 and magnetic amplifier230 is characterized by several inherent advantages, For example, oneadvantage resides in the fact that very low power is required from thephase shift signal control source, i.e the voltage across controlwinding 242, due to the high amplistat gain of magnetic amplifier 230.Another advantage is that control winding 242 can be designed to match avery wide range of signal source impedances. A further advantage is thatthe phase displacement between the outputs of master oscillator 24 andslave oscillator 26 can be made to be the algebraic sum of severalcontrol signals by merely winding several separate control windings onmagnetic amplier 230.

Now referring to the graph of FIG. 8, the abscissa thereof denotescontrol `current of winding 242 in milliamperes and the ordinate thereofdenotes phase displacement between the outputs of oscillators 24 and 26,in electrical degrees. The data for the graph was obtained from theoperation of the circuit embodied in FIGS. 2-4 including oscillators 24and 26 and magnetic amplifier 230.

In bridge inverter 16, returning t FIG, 2, there is con nected betweenjunction 109 wherein the D.C. power input appears and ground, a seriesarrangement of an inductor 250 and the parallel combination of theseries arrangements of silicon controlled rectifiers 252 and 254 andsilicon controlled rectiers 256 and 258 respectively.

The junctions between the cathodes of silicon controlled rectifiers 252256 and the anodes of silicon` controlled rectifiers 254, 258 aredesignated as points 253 and 257, respectively. Connected between thejunction 253 [of the cathode of silicon controlled rectier 252] and thegate electrode of silicon controlled rectier 252 is [the] a seriesarrangement of a secondary winding 260 of transformer 154 and a resistor262. Connected between the cathode and the gate electrode of siliconcontrolled rectifier 254 is [the] a series arrangement of a secondarywinding 264 of transformer 154 and a resistor 266.

Connected between the junction 257 of the cathode of silicon controlledrectifier 256 and the gate electrode of silicon controlled rectifier 256is [the] a series arrangement of a secondary winding 268 of transformer154 and a resistor 270. Connected between the cathode and the gateelectrode of silicon controlled rectifier 258 is the series arrangementof a secondary winding 272 of transformer 154 and a resistor 274.

Connected between junctions 253 and 257 is [the] a primary winding 278of an output transformer 276, primary winding 278 being connected inshunt with a commutating capacitor 282. Connected between the anode ofsilicon controlled rectifier 252 and ground is the series arrangement ofthe cathode to anode paths of diodes 284 and 286, an inductor 291 beingconnected between junction 253 and the junction 285 of the anode ofdiode 284 and the cathode of diode 286, Connected between the anode ofsilicon controlled rectifier 256 and ground is the series arrangement ofthe cathode to anode paths of diodes 288 and 290, an inductor 292 beingconnected between junction 257 and the junction 289 of the anode ofdiode 288 and the cathode of diode 290.

In the operation of bridge inverter 16, it is seen by the designatingpolarity [dots] marks of secondary windings 260, 264, 268 and 272 thatsilicon controlled rectifiers 252 and 258, and silicon controlledrectifiers 254 and 256 are respectively rendered substantiallysimultaneously conductive.

If it is assumed that silicon controlled rectiers 252 and 258 are firstrendered conductive by the supplying of positive current to their gateelectrodes through secondary windings 260 and 272 and through resistors262 and 274 respectively, most of the voltage appearing at junction 109appears across primary winding 278. Such conduction continues for theduration of the half cycle of output from master oscillator 24. Upon theinitiation of the next half cycle of output from master oscillator 24whereby the positive current appears in Secondary windings 264 and 268,capacitor 282 which has been Charged during the preceding half cycle isabruptly connected across silicon controlled rectifiers 252 and 258 inthe reverse polarity, thereby quickly causing silicon controlledrectiers 252 and 258 to cease conducting and to recover their blockingstates respectively. The reverse polarity voltage is applied to siliconcontrolled rectifiers 252 and 258 at a rate which is determined partlyby the load current which is flowing through primary winding 278 andpartly by the series resonant combination of inductors 291 and 292 andcapacitor 282. Conduction now continues in silicon controlled rectifiers254 and 256 and the half cycle of opposite polarity of output isobtained across primary winding 278, etc. Diodes 284 and 286 and diodes288 and 290 are included to permit the returns of energy to the source,i.e., point 109, in conditions such as those of lagging power factorloads, i.e., inductive loads when circulating reactive currents arepresent. Inductor 250 is included to limit the current surge at the timethat commutation occurs from one pair of silicon controlled rectiers tothe other pair of silicon controlled rectifiers.

Bridge inverter 18 is identical to bridge inverter 16 both in structureand in operation. The transformer windings in circuit with the gateelectrodes of silicon controlled rectiers of bridge inverter 18 aresecondary windings of transformer 162 in slave oscillator 26 andaccordingly the output of bridge inverter 18 appearing across [the] aprimary winding 302 of an output transformer 300 is displaced in phasewith respect tn the output appearing across primary winding 278 oftransformer' 276, the same amount as is the displacement in phasebetween the outputs of slave oscillator 26 and master oscillator 24.

It is to be noted that in master and slave oscillators 24 and 26 thattransistors 176 and 220 are simultaneously conductive for the periodthat it takes magnetic amplier 230 to saturate whereupon conductivity isswitched from transistor 220 to transistor 200. Similarly, transistorsand 200 are simultaneously conductively for the period that it takesmagnetic amplifier 230 to saturate at which time conductivity isswitched to transistor 220. Accordingly, in bridge inverter 16, siliconcontrolled rectifiers 252 and 258 are conductive when transistor 1172conducts and silicon controlled rectifiers 256 and 254 are conductivewhen transistor 190 conducts. Likewise, in bridge inverter 18, siliconcontrolled rectifiers 294 and 299 conduct when transistor 200 isconductive and silicon controlled rectifiers 298 and 296 are conductivewhen transistor 220 is conductive. [Thus,] In FIG. 3, the polarities ofserially-connected secondary windings 1 1 280 and 304 of outputtransformers 276 and 300 respectively are such as to provide the properphasor additions of half cycles of like polarity in the outputs ofbridge inverters 16 and 18.

The output filter 22, also shown in FIG. 3, comprises a seriesarrangement of a capacitor 306 and a saturable inductor 308 connectedfrom winding 280 to a ground point 313 and a parallel arrangement of acapacitor 310 and the inductance of that portion 311 of saturabletransformer 312 between terminal 305 of winding 304 and ground point313. Capacitor 306 and inductor 308 are tuned to series resonance at thefrequency of the outputs of oscillators 24 and 26, i.e., the desiredfundamental output frequency and capacitor 310 and inductance 311 aretuned to parallel resonance at the same frequency. Inductor 308 presentsa high impedance to higher harmonics as compared to the impedancepresented by capacitors 306 and 310, and, therefore, has most of theharmonics dropped across it. [Capacitor 310 supplies energy to theoutput during the portion of the cycle when bridge inverters 16 and 18are not enabled] The parallel arrangement of capacitor 3l() andindz/ctance 311 provides additional attenuation of harmonic frequencies,resulting in an output voltage from terminal 305 toI ground point 313which has a substantially sinsoidal waveform from no-load to rated loadconditions.

Inductor 308 is chosen to be of a saturable type and provides a form ofcurrent limiting. Thus, if the current through iuductor 308 exceeds acertain value, it saturates at each half cycle, thereby detuning the LCcircuit cornprising capacitor 306 and inductor 308 and thus droppingmuch of the fundamental, Le., the desired output across it.

The output appearing at point 305 is developed across [a] saturabletransformer 312 which is tapped to ground at about its two-third point.A portion of the output voltage appearing across transformer 312 isfull-wave rectified by diodes 314 and 316 and this rectied voltageappearing at point 323 is applied to the parallel combination comprisinga variable resistor 318 and the series arrangement of the cathode toanode path of a reference Zener diode 320 and a [reistor] resistor322[.] connected to point 3/3. [the] The aforestated control winding 242of magnetic amplifier 230 [being] is connected between the junction 321of diode 320 and resistor 322 and a point 317 on resistor 318.

1t is seen that when the voltage at point 323 is of the proper value,the voltage across control winding 242 and the resultant currenttherethrough are of a normali, quiescent value; the correction voltageresults in a value of phase shift between inverters lo and 18 whichproduces a desired steady-slate output voltage across terminals 305 and313 [there is substantially no voltage developed across control winding242]. When the voltage at point 323 is below the proper value, thevoltage developed on winding 242 is in amplitude and polarity such thatthere is provided increased phase displacement between oscillators 24and 26 and correspondingly increased phase displacement between bridgeinverters I6 and I8. In this condition, the combined output of theinverters 16 and 18 appearing across the polarity mark terminals ofwindings 280 and 304 is increased, due to the oppositelypoled serialconnection thereof [output from magnetic amplifier 230 and the phasedifference between the outputs of oscillators 24 and 26 and consequentlybetween the outputs of inverters 16 and 18 is decreased] When thevoltage at point 323 exceeds the desired value, the voltage developed onwinding 242 is such that there is provided decreased phase displacementbetween oscillators 24 and 26 and thus between bridge inverters 16 and18, resulting in a decreased output appearing across the polarity markterminals of windings 280 and 304 [appearing at point 321 effects thedevelopment of an error voltage on winding 242 in a polarity such as todecrease the output oi magnetic amplifier 230 and thereby to widen thephase displacement between the respective oscillators 24 and 26 and:bridge inverters 16 and 18]. In this manner the A.C. output voltage ofthe system is regulated.

It is to be noted that the voltage appearing at point 321 is not purelya direct current voltage but is a direct current voltage with a smallslice taken out of it each half cycle due to the nature of the voltagewaveform applied. With such arrangement, there is desirably regulatedsubstantially the R.M.S. output voltage rather than the average voltage.

The functions of transformer 312 are to provide a suitable means forfull wave center tapped sensing as applied to diodes 314 and 316. Also,under transient high voltage conditions, transformer 312 saturates,thereby limiting the average output voltage and causing such voltage toreturn to its normal level faster than it would normally so do, therebyproviding voltage clamping action.

It has been stated above that initially the twin cores of transformer154 are respectively orientated in opposite directions. It is thusunderstood that in master oscillator 24, whichever transistor 172 or 190is energized into conduction first, determines the polarity of the rstoutput pulse of oscillator 24. However, regardless of polarity, theduration of the rst output pulse of oscillator 24 is only 90 electricaldegrees due to the fact that one of the cores of transformer 154 isalready at saturation. In effect, therefore, one half of the magneticcircuit in oscillator 24 is not present during the first half cycle andtherefore the duration of the rst half cycle of output of oscillator[154] 24 is only 90 electrical degrees. Each subsequent half cycle ofoutput from oscillator [154] 24 is the normal 180 electrical degrees.

The significance of initially orienting the cores of transformer 154 inopposite directions of orientation when power is applied to the systemcan now be appreciated. Transformers 276 and 300 of bridge inverters 16and 18 represent a very high proportion of the total weight of thesystem (about 50%, depending upon the output frequency). For thisreason, it is desirable to minimize the needed NA, or product of windingturns times effective iron area in these output transformers.Transformers 276 and 300 are suitably designed with a small air gap and,therefore, the flux states thereof respectively at the start of theinitial cycle of operation are close to zero. lf the first part cycle isonly a quarter cycle long, 1'.e., electrical degrees, then therespective fluxes in transformers 278 and 300 reach a maximum fluxdensity condition, say, at state B. If the next half cycle thereafter isnormal, i.e., electrical degrees, the flux is switched in eachtransformer to the state, -B. With succeeding half cycles, the fluxstates of the transformers continue to swing between states -B and +B,etc., and not from zero to 2B as in the case of an ordinary circuit. Forthis reason, it is highly desirable to have the first half cycle ofoperation only one quarter cycle long, such being accomplished aspreviously explained. Since on the first part cycle, regardless of whichtransistor first conducts in oscillator 24, as one core of saturabletransformer 154 is already saturated, the effective required iron areasin the inverter output transformers 276 and 300 respectively are cut inhalf.

In order to insure that no commutation failure can occur at start-up andto insure that the rst part cycle of the output of slave oscillator 26does not exceed 9G electrical degrees, there is included [the] a circuit399 connected between base 196 of transistor 190 and base 226 oftransistor 220. This circuit includes the series arrangement of theanode to cathode path of a diode 400, a resistor 402, the cathode toanode path of a diode 404 and a resistor 406. The junction 403 ofresistor 402 and the cathode of diode 404 is connected to point 146 (thenegative terminal of the regulated D.C. supply) through the parallelcombination of a capacitor 408 and a resistor 410.

In the operation of circuit 399, when current is passed through windings156 and 158 of transformer 154 and the control winding 249 of magneticamplifier 230, the polarity of winding 249 is such that magneticamplifier 230 is saturated during the initial start-up transient. Diode400, resistor 402 and resistor 410 insure that transistor 190 in masteroscillator 24 is the iirst to be rendered conductive and resistors 406and 410 and diode 404 insure that transistor 220 is the iirst to berendered conductive in slave oscillator 26. With this arrangement atstart-up, silicon controlled rectiliers 252 and 258 in bridge inverter16 and silicon controlled rectifiers 294 and 299 in bridge inverter 18are also substantially simultaneously rst rendered conductive.

The polarity of primary winding 302 of output transformer 300 as shownby the designating polarity [dot] mark thereon is chosen such that atthe initial start-up transient, minimum voltage occurs at the Output[terminals] of the system, i.e., the phasor sum of the voltage insecondary windings [276] 280 and 304. This can be understood when it isrealized that since initially the voltage outputs of master and slaveoscillators 24 and 26 and consequently the outputs of inverters 16 and18 are in unison due to the `action of control winding 249 and start-upcircuit 399, the polarities of windings [276] 280 and 304 are such thatthe voltages appearing therein oppose each other. After the first partcycle, the voltage across control winding 242 of magnetic amplifier 230at first is of an amplitude and polarity such as to maintain a graduallydecreasing output from magnetic ampliiier 230 whereby a phase differencedevelops between the outputs of oscillators 24 and 26 and the phasor sumof the voltages in windings [276] 280 and 304 gradually increases. Withthis arrangement the system output voltage builds up smoothly until thedesired output voltage and transient overshoot of the output voltageduring initial start-up is substantially eliminated.

In FIG. 6, there is shown another embodiment of an arrangementcomprising a master-slave oscillator with a magnetic phase shiftercoupling therebetween. In this figure, there is shown a iirst magneticcoupled multivibrator 330 comprising transistors 332 and 340 and asaturable transformer 350 and a second magnetic coupled multivibratorcomprising transistors 362 and 370 and a transformer 380. Multivibrator330 has a natural frequency which is the desired frequency. The outputof multivibrator 360 is synchronized with and displaced in phase fromthe output of the multivibrator 330.

In multivibrator 330, transistor 332 has its emitter 336 connected tothe positive terminal 333 of a unidirectional potential source 334 andits collector 338 connected to the negative terminal 335 of source 334through a primary winding 352 of saturable transformer 350. The base 339of transistor 332 is connected to a common junction 357 [positiveterminal 333] through a secondary winding 353 of transformer 350. [and]Junction 357 in zum is connected lo positive terminal 333 through aresistor 358 and [is connected] to negative terminal 335 through aresistor 359.

The other transistor 340 of the multivibrator 330 has its emitter 342directly connected to terminal 333, its collector 344 connected tonegative terminal 335 through a primary winding 354 of transformer 350,and its base 346 connected to junction 357 through a secondary winding355. Saturable transformer 350 may suitably be an autotransformer andcomprises a core preferably of a grain oriented magnetic metal having agiven volt-second characteristic.

Multivibrator 360 is essentially similar to the multivibrator 330 exceptthat transformer 380 therein need not be of a saturable type, i.e., itscore need not be of a grain oriented material. If it is of the saturabletype, then, of course, its volt-second characteristic has to be greaterthan transformer 350 in multivibrator 330 [a suitable difference in suchvolt-second characteristic being about 25 percent as has been explainedabove]l With such difference when transformer 380 is of the saturabletype, then the natural frequency of multivibrator 360 is less than thatof multivibrator 330.

In multivibrator 360, transistor 362 has its emitter 364 connected topositive terminal 333, its collector 366 connected to negative terminal335 through a primary winding 382 of transformer 380 and its base 368connected to [positive terminal 333] a common junction 387 through asecondary winding 383 of transformer 380. [and] Junction 387 in turn isconnected to positive terminal 333 through a resistor 388[,] and[connected] to negative terminal 335 through a resistor 389. Transistor370 has its emitter 372 directly connected to positive terminal 333, itscollector 374 connected to negative terminal 335 through a primarywinding 384 of transformer 380 and its base 376 connected to junction387 through a secondary winding 385 of transformer 380.

A secondary winding 351 of transformer 350 has its polarity [dot] markterminal connected to base 376 of transistor 370 and 1o one side ofvariable rcrisior 392 and and its other terminal connected to base 368of transistor 362 through [a] Ille other side and tap of variabieresistor 392 and a saturable reactor 394. The designating polarity[dots] marks on the windings of transformers 350 and 380 show thedirection of current ow therethrough to produce positive ampere turnstherein.

Considering the operation of multivibrators 330 and 360 of FIG. 6 andthe coupling therebetween comprising secondary winding 351, variableresistor 392 and saturable reactor 394, if it is assumed thattransistors 332 and 370 are conductive, that the voltages at thepolarity [dot] mark terminals of the windings of transformer 350 arepositive and that the voltages at polarity [dot] mark terminals of thewindings of transformer 380 are negative, in such situation, it is seenthat the entire voltage across secondary windings 383 and 385 oftransformer 380 is added to a portion of the voltage in secondarywinding 351 of transformer 380, such portion being determined by thevalue of the portion of variable resistor 392, and the combination ofthese two voltages is applied to saturable reactor 394. If it is assumedthat initially inductor 394 is at negative saturation, i.e., itsmagnetic ux is so oriented as to require exciting current owtherethrough in the direction from base 368 to variable resistor 392, afixed predictable time elapses before reactor 394 abruptly saturates inaccordance with the following equation:

wherein N is the amount of turns on reactor 394, A is the effective ironarea in square inches in reactor 394, Bs is the saturation flux densityin lines per square inch in reactor 394, and E is the total voltageapplied to reactor 394.

At the instant that reactor 394 saturates, the potential at base 368goes rapidly in the negative direction to switch transistor [360] 362into conductivity and the 0pposite half cycle of output frommultivibrator 360 is produced.

Since transformer 380 is either of the unsaturable type or if of thesaturable type is chosen to have an NABs product which is appreciablygreater than the NABs product of transformer 350, the switching periodof multivibrator 360 is determined by the volt-second characteristic ofreactor 394 and the voltage applied thereto as determined in part by thevalue of the portion of resistor 392. The volt-second characteristic of[inductor] reactor 394 consequently determines the amount of phasedisplacement between the output of multivibrator 330 and the output ofmultivibrator 360.

It is to be noted that reactor 394 has to be chosen to have avolt-second characteristic such that its time of switching fromsaturation in one direction to saturation in the opposite directioncannot exceed the time of a half cycle of output from multivibrator 330as has been previously explained in connection with magnetic amplifier230 in FIGS. 2 4. [If its volt-second characteristic is chosen such thatits saturation time might be longer than the period of such half cycleof output from multivibrator 33t), then, of course, the frequency of theoutput of multivibrator 360 in the event that transformer 380 were ofthe saturable type would be its natural frequency as determined by thevolt-second characteristic of transformer 38()` and the value ofpotential source 334. In this latter type situation, multivibrator 330could not Control the output frequency of multivibrator 360.]Accordingly` with the arrangement of the circuit of FIG. 6, the phasedifference permitted between the outputs of both multivibrators is up toa maximum 180.

[It is, of course, further to be noted that if the volt- Secondcharacteristic of transformer 380, in the event that it were saturablewere equal to or less than the voltsccond characteristic of transformer350, multivibrator 360 would have its natural output frequencyindependent of the frequency of the output of multivibrator 330. Ifsaturable reactor 394 were eliminated from the circuit and iftransformer 380 were either of the unsaturable type or the saturabletype and having a greater voltsecond characteristic than transformer350, the output of multivibrator 360 would be in synchronisrn with theoutput of multivibrator 330 with no phase difference therebetween]Furthermore, as has been explained with reference fo FIGS. 2 4, thevolt-second characteristic 0j transformer 380 in .slave multivibrator360 mast be larger ltlian that f transformer 350 in mast-ermultivibrator 330 in order to achieve proper operation.

Resistor 392 may be 'utilized to vary the volt-second capabilities ofsaturable reactor 394 whereby its time of saturation may range from aminimal period to a period equal to the time of a half cycle of outputfrom multivibrator 330.

[In the graph of FIG. 8, the abscissa is control current in milliamperesand the ordinates are phase displacement in electrical degrees. The datafor the graph is obtained from the operation of the portion of thecircuit of FIGS. 2 4, which includes oscillators 24 and 26 and magneticamplifier 230. The outputs of oscillator 26, FIGS. 2 4 and multivibrator360 in FIG. 6 have been found to be substantially distortion free.]

A combination such as that comprising master oscillator 24, and slaveoscillator 26 and magnetic amplifier 230 shown in FIGS. 2-4 or acombination such as that of multivibrators 330 and 360 and saturablereactor 394 shown in FIG. 6 provide arrangements whereby there may beproduced a plurality of [rectangular] square i wave [signals] voltageswhich are displaced in phase with respect to each other for varyingamounts. These combinations may accordingly be used advantageously forcontrolling the output voltage of. an inverter system by connecting theoutputs of the two inverters in series [arrangement] arrangement andcontrolling the total output voltage therefrom by phase shifting theoutput of one inverter with respect to the other. Such combinationsovercome the disadvantage of a resistance-capacitance phase shiftcircuit in that the output waveform is not distorted and smooth controlof such phase shifting is readily attained automatically.

In addition, a resistance-capacitance phase shift network does notenable smooth phase shifting and generally requires the need of theintervention of an operator to vary a resistance or a capacitance bysuitable manual means to effect the change in phase shift.

In FIG. 7, there is shown an application of a circuit comprising amaster oscillator, a slave oscillator and a magnetic phase shiftercoupling therebetween to effect voltage regulation of a static invertercircuit or the serially combined outputs a plurality of two staticinverter circuits.

In this circuit, the input power source 410 which may be aunidirectional potential source is applied to a voltage regulator 412and is also applied to a power switching stage 414 and a power switchingstage 416. Oscillators 418 and 42() which are magnetic coupledmultivibrators such as oscillators 24 and 26 in FIGS. 2-4 provide thesquare wave switching voltages for power switching stages 414 and 416respectively. The capacitors 418C and 420C serve to provide relativelyrapid switching of conductivity in one transistor to the othertransistor in oscillators 418 and 420 respectively thereby aiding inproviding sharp, rectangular `wave outputs therefrom.

The output of voltage regulator 412 is also applied to an isolationamplifier 419 comprising a transistor 422 and a transistor 432. [In thelater circuit, transistor] Transistor 422 has its emitter 424 connectedto the positive terminal of the output from regulator 412 and itscollector 426 connected to the negative terminal of regulator 412through a primary winding 442 of a transformer 440. The base 428 oftransistor 422 is connected to the junction 430 of emitter 424 and thepositive terminal of voltage regulator 412 through [the] a seriesarrangement of a secondary `winding 418 TS1 of transformer 418T inoscillator 412 and a resistor 431.

`Transistor 432 has its emitter 434 connected to junction 430, its base438 connected to junction 430 through the series arrangement of asecondary winding 418 TS2 of transformer 418'? and a resistor 441 andits collector 436 connected to the junction 443 of winding 442 oftransformer 440 and the negative terminal of voltage regulator 412through a primary winding 444 [winding] of transformer 440. The anode tocathode path of a diode 429 is provided connected between collector 426and emitter 424 of transistor 422 and the anode to cathode path of adiode 439 is provided connected between collector 436 and emitter 434 oftransistor 432.

In the operation of the isolation amplifier 419 comprising transistors422 and 432 and their associated circuit components, it is seen by thedesignating polarity [dots] marks on secondary windings 418 TS1 and 418TS2 of transformer 418i, that bases 428 and 438 are alternately drivenin the negative direction in accordance with the switching intoconductivity of transistors 418A and 418B of oscillator 418.Accordingly, isolation amplifier 419 provides an output ai collectors426 and 436 una' across windings 442 and 444 which is in exactsynchronism with the output of oscillator 418 with no phase displacementbetween their respective outputs. Diodes 429 and 439 are included toprovide transient suppression in accordance with well known practices.

Power switching stages 414 and 416 may suitably contain devices such assilicon controlled rectifiers which are rendered alternately conductivein accordance with the square wave voltages applied thereto fromosillators 418 and 420 respectively whereby there is provided at theoutputs of stages 414 and 416, square wave outputs in accordance withthe outputs of oscillators 418 and 420.

A combining network and filter stage 450 may suitably comprise means forserially combining the outputs of power switching stages 414 and 416,such combining means suitably being secondary windings of respectiveoutput transformers in the power switching stages 414 and 416 connectedin series and the filter portion of stage 450 may suitably be a low passlter for converting the combined quasi rectangular wave outputs to arealtively pure sinusoidal form. The output of the circuit is taken fromcombining network and filter stage 450.

Slick output is applied t0 a comparison network 452 through a rectifier451 which may compi-irc means such as diodes 314 and 316 of FIG. 3. [thecomparison network comprising two parallel arms] One parallel arm ofnetwork 452 comprises [the] a series arrangement of a resistor 455 andthe cathode to anode path of a reference diode 454 such as a Zenerdiode, thc anode of diode 454 being connected to a neutral. [and theother] 1 7 Another parallel arm comprises `a series arrangement of aresistor 456 and a variable resistor 458. Across diode 454 there isdeveloped the proper voltage against which the output voltage isreferenced.

A control winding 462 of a self-saturating magnetic amplifier 460, i.e.,an amplistat, has its polarity [dot] mark terminal 461 connected to thecathode of reference diode 454 and its other terminal connected by meansof a tap to a point 463 on variable resistor 458, there being developedon control winding 462 a correction voltage which is the differencebetween the output voltage of stage 450 and the voltage across referencediode 454. Control winding 462 encompasses both cores of twin coremagnetic amplifier 460, amplifier 460 also comprising gate windings 464and 466. Terminals 465 and 467 of gate windings 464 and 466 respectivelyare connected together, the junction thereof being connected to the baseof transistor 420B in the oscillator 420. The other terminalsrespectively of gate windings 464 and 466 are connected through theanode to cathode path of a diode 468 and the anode to cathode path of adiode 470, the junction 469 of the cathode of diode 468 and the anode ofdiode 470 being connected to the base of transistor 420A through asecondary winding 446 of transformer 440.

In considering the operation of the system of FIG. 7, it is seen thatthe [difference] correction voltage developed on control winding 462 isthe phase shift control signal for oscillator 420. Winding 446 serves asa combining means for the voltage appearing across windings 442 and 444in isolation amplifier 419 and the voltage provided from magneticamplifier 460. Accordingly, it is seen that the output of oscillator 420is synchronized frequency wise with the output of oscillator 418, butthat its output is displaced in phase with respect to the output ofoscillator 418 depending upon [he] the volt-second characteristic ofmagnetic amplilier 460 and the amplitude of the control signal appliedto control winding 462. [Thus, in the event that the difference voltagedeveloped on control winding 462 in the positive ampere turns directionis a relatively large one whereby the phase displacement between theoutputs of oscillators 418 and 420 and, consequently, the outputs ofpower switching stages 414 and 416 are relatively small, the output ofcombining network 450 will be correspondingly increased as a consquencethereof and vice versa] Thus, a fluctuation in the system output voltageresults in adjustment of the relative phase shift between oscillators418 and 420 by means of magnetic ampliyier 460; by proper serialtransformer connections within the network 450, as with the secondarywindings 280 and 304 in FIG. 3, regulating of the system output voltageas previously described can be obtained. It is to be realized that themaximum phase displacement between the outputs of oscillators 418 and420 cannot exceed 180.

In FIG. 5 wherein there is shown another illustrative embodiment of afrequency changer in accordance with the principles of the invention, amultiple phase input such as a three phase input which is randomlyvariable in voltage and frequency is passed through a low pass filter500. Filter 500 serves to prevent radio interference generated by thebridge rectifier 502 from flowing back into the input power source andto filter any random high voltage spikes which may occur in thealternating current power supplied to bridge rectifier 502 to therebyeliminate the possibility of rectifier damage which might resultotherwise from random input transients.

The output of filter 500 is [rectifier] rectified in bridge rectifier502 directly without the use of an input transformer. Rectifier 502 maysuitably comprise a three phase double-way bridge rectifier whereinsteady state voltage ratings are selected such as to permit safeoperation during transients up to a chosen value R.M.S. line to neutral.

The output of bridge rectifier 502 is filtered in a D.C. filter 504,filter 504 suitably being an LC choke input lter which smooths theoutput from bridge rectifier 502 to eliminate voltage modulation[s] ofthe output of the system. The L to C ratio in lter 504 is chosen to bcsmall to minimize voltage transients due to step changes in the outputload of the system.

The smooth but unregulated output from the filter 504 is applied to apower inverter circuit 506, This circuit contains switching devices suchas high current silicon controlled rectiliers in a bridge inverterconnection. Circuit 506 may also contain silicon controlled rectiers forcontrolling its output voltage, such control being enabled by theeffecting of independent control of commutation of the high currentsilicon controlled rectifiers. Power inverter circuit 506 may alsocontain commutation components comprising capacitors and inductors whichprovide resonant discharge paths so that the commutation intervalbetween the high current, i.e., the load carrying, silicon controlledrectifiers is essentially independent of the electrical load on thesystem. These inductors may be tapped in a manner such that the chargestored in the commutating capacitors is a function of load currentwhereby commutation efliciency is high both for very light and heavyloads.

There may also be included in power inverter circuit 506, pump backrectifiers, i.e., rectificrs which are utilized to prevent commutationfailures due to reactive loads and which form part of the commutationcircuit, these rectiliers also permitting the flow of energy from the AC. load back to the D.C. supply as may be required for lagging powerfactors. The frequency of the output of power inverter circuit 506 iscontrolled by a square wave voltage having the desired frequency of theoutput of the system and which is applied to stage 506 together with theD.C. power output from filter 504.

The output voltage waveform as seen across, for example, an outputvoltage transformer included in stage 506 consists of alternating squarepulses of relatively constant amplitude, and whose widths depend uponthe periods of conduction of the high current silicon controlledrectifiers. Such waveform may be designated a quasi square wave."

The output of power inverter circuit 506 is ltered in A.C. filter stage508. Filter 508 is suitably of the socalled fourth order type aspreviously hereinabove described in connection with the system of FIGS.1-4 and provides a sine wave output.

The output of radio interference filter stage 50() is also applied to aregulator 510 which provides a regulated relatively low power D.C. powersupply for a square wave multivibrator 512. Regulator 510 may suitablybe a selfsaturating `magnetic amplifier, i.e., an amplistat," comprisinga plurality of gate windings and a plurality of control windings. One ofthese control windings has applied thereto a D.C. signal which controlsthe amplitude of the output of regulator 510, i.e., a unidirectionalpotential which substantially has the value desired for the D.C. supplyfor the square wave multivibrator 512. A reference voltage of the propervalue may be developed across a Zener reference diode in referencevoltage source 514 and such reference voltage is compared with theoutput of regulator 510 in stage 515r` the error signal resulting fromsuch complarision providing the control signal for the control windingin the magnetic amplifier of regulator 510 whereby there is produced atthe output of 510, a regulated D.C. supply for square wave multivibrator512.

Square wave multivibrator 512 is suitably a multivibrator whose outputfrequency is a function of its supply voltage and may be a magneticcoupled multivibrator. The frequency of multivibrator 512 is chosen tobe the desired output frequency of the system. The square-uf'ave voltageoutput of multivibrator 512 is applied as an input to power inverterstage 506 to provide gating signals for the high current siliconcontrolled rectiiiers therein. The gating circuits associated with thesilicon controlled rectifiers of power inverter 506 are designed wherebya negative gate bias voltage is applied to all of the silicon controlledrectifiers except when positive gating pulses are actually beingsupplied thereby eliminat- 19 ing any possibility of false triggering"such as may occur with gating circuits which are not designed to providenegative bias,

The output voltage provided from A.C. filter S08 is applied to a voltagesensing and voltage adjusting circuit 517. In circuit 517, the outputvoltage is rectified to obtain a D.C. voltage whose value isproportional to the average of the A.C. voltage at the output of filter508. Such D.C. voltage is then [compard,] compared as depicted inelement 518, with the voltage developed across a Zener reference diodein reference voltage source `516. Any difference, i.e., error voltagegenerated as a consequence of such comparison is applied to a controlwinding of an output voltage regulator amplistat 520. Amplistat 520controls the gating signals to the voltage regulating silicon controlledrectifiers in power inverter 506 such that control current changes inamplistat 520 results in rapid and accurate control of the quasi-squarewave output of power inverter 506. Thereby, there is regulated thevoltage of the sine wave output of filter 508. Amplistat 520 may alsocontain a separate control winding shunted by resistor and inductor toachieve lag-lead" compensation of the frequency response characteristicof the amplistat. The resistor and inductor in series with such controlwinding is so designed as to optimize transient response of the systemto input line voltage fiuctuation, Le., the input to radio interferencefilter 500, as well as to abrupt output load changes.

[While there have been shown particular embodiments of this invention,it will, of course, be understood that it is not wished to be limitedthereto since different modifications may be made both in the circuitarrangements and in the instrumentalities employed, and it iscontemplated in the appended claims to cover any such modifications asfall within the true spirit and scope of the invention] What is claimedas new and desired to be secured by Letters Patent of the United Statesis:

1. In combination with a power source for producing a polyphase outputhaving a randomly variable voltage and frequency, polyphase rectifyingmeans in circuit with said source for converting said polyphase outputto a single substantially unidirectional power signal, power switchingmeans, an oscillator having a chosen frequency, means in circuit withsaid power source for deriving a unidirectional voltage of a given valuetherefrom, means for applying said unidirectional voltage as a supplyvoltage to said oscillator, means for applying said unidirectional powersignal and the output of said oscillator to said power switching meansto produce a single phase power output having said chosen frequency, areference voltage source, means in circuit with said power switchingmeans and said reference voltage source for comparing the voltage ofsaid single phase power output with said reference voltage to produce adifference voltage therebetween, and means for applying said differencevoltage as an error voltage to said power switching means.

2. In combination with a power source for producing a polyphase outputhaving a randomly variable frequency and voltage, polyphase rectifyingmeans in circuit with said polyphase source for converting saidpolyphase output to a single substantially unidirectional power signal,power switching means comprising first and second power invertersconnected in bridge arrangement, first and second oscillators, means incircuit with said power source for driving a unidirectional voltage of agiven value therefrom, means for applying said unidirectional voltage tosaid first and second oscillators respectively, phase shifting means forcoupling the output of said first oscillator to the input of said secondoscillator to produce outputs from said first and second oscillatorshaving the same frequency but being displaced in phase with respect toeach other an amount in accordance with a voltage applied to said phaseshifting means, means for applying the output of said first oscillatorand said single phase CII power signal as inputs to said first powerinverter to produce an output from said first inverter having thefrequency and being in phase with the output of said first oscillator,means for applying said single phase power signal and the output of saidsecond oscillator to said second power inverter to produce a poweroutput from said second inverter having the frequency and being in phasewith the output of said second oscillator, means in circuit with theoutput of said inverters for vectorially combining the outputstherefrom, a reference voltage source, means in circuit with said lastnamed source and said combining means for comparing the voltage of saidcombining means output with said reference voltage to produce adifference voltage therebetween, and means for applying said differencevoltage to said phase shifter.

3. In combination with a power source for producing a polyphase outputhaving a randomly variable voltage and frequency, polyphase rectifyingmeans in circuit with said polyphase source for converting saidpolyphase output to a single substantially unidirectional power signal,power switching means comprising first and second power inverters, meansin circuit with said power source for deriving a unidirectional voltageof a given value therefrom, a first reference voltage source, means incircuit with said deriving means and said first voltage source forcomparing said derived voltage with said reference voltage to produce afirst difference voltage therebetween, means for applying said firstdifference voltage to said deriving means to produce a regulatedunidirectional derived voltage having a chosen value, a first magneticcoupled multivibrator comprising a saturable transformer having a givenvolt-Second characteristic, means for applying said regulated derivedvoltage as a supply voltage to said first multivibrator to produce anoutput from said first multivibrator having a frequency which is thefunction of the magnitude of said derived voltage and said volt-secondcharacteristic, a second magnetic coupled multivibrator, means forapplying said regulated derived voltage as a supply voltage to saidsecond multivibrator, magnetic phase shifting means for applying theoutput of said first multivibrator as a driving signal to said secondmultivibrator to produce an output from said second multivibrator havingthe frequency of the output of the first multivibrator but displaced inphase therefrom, said magnetic phase shifting means comprising meanshaving a prescribed volt-second characteristic and which is saturable ina period which is a function of said volt-second characteristic andmagnitude of a voltage applied thereto, means for applying the output ofsaid first multivibrator and said unidirectional power signal to saidfirst power inverter to produce an output therefrom having the frequencyof said multivibrators and in phase with the output of said firstmultivibrator, means for applying the output of said secondmultivibrator and said unidirectional power signal to said second powerinverter to produce an output therefrom having the frequency of saidmultivibrators and displaced in phase with respect to the output of saidfirst power Inverter the same amount as the phase displacement betweensaid first and second multivibrators, means in circuit with said powerinverters to vectorially combine the outputs therefrom, a secondreference voltage source, means in circuit with said combining means andsaid second source for comparing the voltage of. the output of saidcombining means with said second reference voltage to produce a seconddifference voltage therebetween, and rneans for applying said secondvoltage as the voltage for said magnetic phase shifter.

4. In combination with a power source for producing a polyphase outputhaving a randomly variable frequency and voltage, polyphase rectifyingmeans in circuit `with said polyphase source for converting saidpolyphase output to a single substantially unidirectional power signal,power switching means comprising first and second power inverters, meansin circuit with said power source for deriving a voltage therefrom, amagnetic amplifier comprising control and gate means, means for applyingsaid derived voltage to said gate means, a first reference voltagesource, means in circuit with said magnetic amplifier and said firstsource for comparing the voltage output from said magnetic amplifierwith said first reference voltage to provide a first difference voltagetherebetween, means for applying said first difference voltage to saidcontrol means to produce a regulated voltage at the output of saidmagnetic amplifier, a first magnetic coupled multivibrator comprising asaturable transformer having a prescribed volt-second characteristic,means for applying said regulated voltage to said first multivibrator toproduce an output therefrom having a frequency in accordance with theamplitude of the said regulated voltage and said volt-secondcharacteristics, a second magnetic coupled multivibrator, magnetic phaseshifting means for applying the output of said first multivibrator as adriving signal to said second multivibrator to produce an output fromsaid second multivibrator having the frequency of said firstmultivibrator but displaced in phase therefrom, said magnetic phaseshifting means comprising saturable means having a predeterminedvolt-second characteristic `whereby said magnetic phase shifting meansis saturable in a period which is in accordance with its volt-secondcharacteristic and a voltage applied thereto, means for applying theoutput of said first multivibrator and said unidirectional power signalas inputs to said first power inverter to produce a power outputtherefrom having the frequency of said multivibrators, means forapplying the output of said second multivibrator and said unidirectionalpower signal to said second power inverter to produce an outputtherefrom having the frequency of said multivibrators but displaced inphase with respect to the output of said first power inverter the sameas the displacement in phase between the outputs of first and secondmultivibrators, means for vectorially combining the outputs of saidpower inverters, filter means in circuit with the output of saidcombining means for substantially removing therefrom components having afrequency other than the frequency of said multivibrator outputs, asecond reference voltage source, means in circuit with said secondvoltage source and said filter means for comparing the voltage of theoutput of said filter means `with said second reference voltage toderive a second difference voltage therebetween, and means for applyingsaid second difference voltage as the voltage for said magnetic phaseshifting means.

5. In the combination defined in claim 4 wherein means are included forderiving said second reference voltage from the output of said combiningmeans.

6. In the combination defined in claim 4 wherein said magnetic phaseshifting means comprises a saturable reactor coupling said first andsecond multivibrators.

7. In the combination defined in claim 4 wherein said magnetic phaseshifting means comprises a magnetic amplifier having gate means couplingsaid first and second multivibrators and control means, said seconddifference voltage being applied to said last named control means.

8. In the combination defined in claim 7 wherein each of said powerinverters comprises an output transformer and wherein the saturabletransformer of said first oscillator comprises a pair of like cores, afirst winding around one of said cores in one polarity, a second windingaround the other of said cores in the opposite polarity, a plurality ofprimary and secondary windings encompassing both of said cores and meansfor initially applying said regulated voltage to said first and secondwindings whereby said cores are initially saturated in oppositedirections, there thereby being produced initially from said firstoscillator, a part cycle which is less than 180 electrical degrees.

9. In the combination defined in claim 8 wherein said magnetic amplifierof said magnetic `phase shifting means includes first control means towhich said second difference voltage is applied and second controlmeans, said second control means being so poled whereby said magneticphase shifting means is initially saturated, and wherein each of saidmultivibrators comprises a pair of active devices which are conductiveduring alternate half cycles, said combination further including meansin circuit with achosen active device of each of said oscillatorsrespectively for insuring that said chosen active devices are the firstto be rendered conductive to produce outputs from said first and secondmultivibrators which are initially in phase.

10. In the combination defined in claim 4 wherein each of said powerinverters comprise a pair of inverter elements in bridge arrangement.

11. In the combination defined in claim 10 wherein each of said powerinverter elements comprise first and second silicon controlledrectifiers, each of said silicon controlled rectifiers being alternatelygated into conductivity in response to a half cycle of output of thesame polarity from a multivibrator, a first silicon controlled rectifierof one power inverter element and a second silicon controlled rectifierof the other power inverter element being substantially simultaneouslyrendered couductive.

12. In the combination dened in claim 4 wherein said filter meanscomprises a first series combination, tuned to said frequency, of afirst capacitor and a first saturable inductor in series arrangementwith the output of said combining means and a second parallelcombination, tuned to said frequency, of a second capacitor and a secondsaturable inductor connected in parallel with the out- .put of saidcombining means, said saturable inductors saturating at chosen currentlevels to detune said resonant combinations.

13. In the combination defined in claim 12 wherein said filter meansfurther includes a saturable output transformer, a portion of which issaid second inductor, and across which the output of said filter meansis developed, said last named transformer saturating at overvoltages.

14. In the combination with a power source for producing a polyphaseoutput having a randomly variable voltage and frequency, polyphaserectifying means in circuit with said polyphase source for convertingsaid polyphase output to a single substantially unidirectional powersignal, power switching means, means for deriving a voltage from saidpolyphase output, a first magnetic amplifier comprising first controland first gate means, means for applying said derived voltage to saidgate means, a first reference voltage source, means in circuit with saidfirst source and said first magnetic amplifier for comparing the outputof said first magnetic amplifier with said first reference voltage toprovide a first difference voltage therebetween, means for applying saidfirst difference voltage to said rst control means to provide at theoutput of said first magnetic amplifier a regulated derived voltage, amagnetic coupled multivibrator comprising a saturable transformer havinga given volt-second characteristic, means for applying said regulatedderived voltage as a supply voltage to said multivibrator to produce asquare wave having a frcquency which is in accordance with the amplitudeof said derived voltage and said volt-second characteristic, means forapplying the output of said multivibrator and lsaid power signal to saidpower switching means to produce a power output having the frequency ofsaid multivibrator, filter means in circuit with said power switchingmeans for substantially removing from the output thereof componentsother than those having the frequency of said multivibrator, a secondreference voltage source, means in circuit with said second source andsaid filter means for comparing the voltage of the output of said filtermeans with said second reference voltage to derive a second differencevoltage therebetween, a second magnetic amplifier comprising second gateand second control means, means for applying the output of saidmultivibrator to said second gate means and for applying said seconddifference voltage to said second control means to provide an outputfrom said second magnetic amplifier which is in accordance with saidsecond difference voltage, and means for applying the output of saidsecond magnetic amplifier to said power switching means as an errorvoltage.

15. In the combination defined in claim 14 wherein said filter meansincludes a series connected first saturable inductor and a firstcapacitor in series arrangement with the output of said power switchingmeans and a parallel connected second capacitor and a second saturableinductor connected across the output of said power switching means, saidinductors and capacitors being respectively tuned to the frequency ofsaid multivibrator, said inductors saturating when the current in saidpower switching means output exceeds a predetermined value.

16. In the combination defined in claim 15 wherein said filter meansfurther includes a saturable output transformer, a portion of which issaid second inductor, and across which the output of said filter meansis developed, said saturable transformer saturating at overvoltages tothereby limit the periods and magnitudes of overvoltage transients.

[17. In combination, a source of rectangular wave voltage having anatural frequency, a rectangular wave oscillator, gate means which isswitched from the substantially nonconductive to the substantiallyconductive state in response to the volt-seconds applied thereto, thetime required for such switching being a factor of the magnitude of saidapplied volt-seconds, and means for applying said rectangular wavevoltage as a driving signal to said oscillator through said gate meansto produce an output from said oscillator having said frequency, theoutput of said oscillator being displaced in phase with respect to thephase of said source voltage in accordance with said time required] [18.The combination defined in claim 17 wherein said source comprises a pairof like active devices and magnetic means for coupling the respectiveoutputs from each of said devices to the inputs of the other of saiddevices, said coupling means comprising a first saturable transformerhaving a predetermined volt-second characteristic whereby said naturalfrequency of the output of said source is a function of saidcharacteristic] [19. The combination defined in claim 18 wherein saidoscillator comprises a pair of active devices and means for coupling theoutputs of each of said respective devices to the inputs of the otherdevices, said coupling means comprising a second transformen] [20. Thecombination defined in claim 19 wherein said second transformer issaturable and has a volt-second characteristic which is greater thansaid volt-second characteristic of said first transformer whereby saidnatural frequency of said source is greater than the natural frequencyof said oscillator] 21. In combination, a first rectangular waveoscillator comprising a pair of first active devices, a saturabletransformer for coupling the outputs of each of said devicesrespectively to the inputs of the other devices, said transformer havinga chosen volt-second characteristic whereby the natural frequency ofsaid first oscillator is a function of said characteristic, a secondrectangular wave oscillator comprising a pair of second active devicesand means for coupling the outputs of each of said Second devicesrespectively to the inputs of the other devices, a magnetic amplifiercomprising control winding means and gate winding means coupling saidoscillators, said magnetic amplifier having a given volt-secondcharacteristic, an electric signal source in circuit arrangement withsaid control winding, and means for applying the output of said firstoscillator to said second oscillator through said magnetic amplifier asa driving signal for said Second oscillator to produce an output fromsaid second oscillator having the frequency of the output of said firstoscillator, said output of said second oscillator being displaced inphase with respect to the output of said first oscillator an amountwhich is proportional to the volt-second characteristic of said magneticamplier and the magnitude of said control signal.

22. In the combination defined in claim 21 wherein the coupling means ofsaid second oscillator comprises a transformer.

23. In the combination defined in claim 21, wherein the coupling meansof said second oscillator comprises a saturable transformer having avolt-second characteristic which is greater than the volt-secondcharacteristic of the saturable transformer in said first oscillator.

24. In the combination defined in claim 23 wherein said magneticamplifier comprises two cores and wherein said gate winding meanscomprises two windings, each of said windings being in seriesarrangement with a rectifier, the junction of said rectifiers beingcoupled to the input of one of the active devices in said secondoscillator, the junction of said windings being coupled to the input ofthe other of said devices in said second oscillator.

25. In an inverter wherein power from a unidirectional current source isconverted into alternating current power of a chosen frequency, meansfor regulating the output voltage of said alternating current powercomprising a first rectangular wave oscillator having said chosenfrequency for controlling the frequency of the output of said inverter,said first oscillator comprising a pair of first active devices andsaturable transformer means for coupling the respective outputs of saiddevices to each other, means for applying a voltage derived from saidsource as a supply voltage to said first oscillator, a second oscillatorcomprising a pair of second active devices and means for coupling theoutputs of said devices respectively to the inputs of the other devices,means for applying said derived voltage as a supply voltage to saidsecond oscillator, first and second power switching means, means forapplying the output of said source to said first and second switchingmeans, means for applying the output of said first and secondoscillators to said first and second power switching means respectively,the outputs of said first and second power switching means beingalternating current power outputs respectively having said chosenfrequency, means for serially combining the outputs of said powerswitching means, means in circuit with the output of said last namedcombining means for deriving a voltage of a chosen value therefrom andfor comparing the output voltage of said combining means with saidderived voltage to produce a difference voltage therebetween, saturableswitching means comprising control means and gate means, means forapplying said difference voltage to said control means, means forapplying the output of said first oscillator through said saturableswitching means as a driving signal to said second oscillator, theoutputs of said second oscillator and said second power switching meanshaving said chosen frequency but being displaced in phase with respectto the outputs of said first oscillator and first power switching meansan amount `which is inverse to the magnitude of said difference voltagewhereby the voltage of said output combining means is regulated.

26. In combination with a power source producing an output having avariable voltage and frequency, rectfying means in circuit wllz saidsource for converting said output to a single substantiallyunidirectional power signal. power switching means, an oscillator havinga chosen frequency,

means in circuit with Said power source for deriving a unidirectionalcontrol voltage of a given value therefrom,

means for applying said unidirectional voltage 10 said 25 oscillator toestablish the pre-determined oscillator' frequency,

means for applying said unidirectional power signal and the output ofsaid oscillator to said power switching means to produce an outputhaving said chosen frequency, a reference voltage source,

means in circuit with said power switching means and said referencevoltage source for comparing the voltage of said output with saidreference voltage to produce a difference voltagetherebetween and,

means for applying said difference voltage as an error voltage to saidpower switching means.

27. In combination with a power source for producing an output having avariable frequency ana' voltage, rectifying means in circuit with saidsource for converting said output to a single substantiallyunidirectional power signal, power switching means comprising first andsecond power inverters connected in bridge arrangement, frst and secondoscillators, means in circuit with said power source for driving aunidirectional voltage of a given value therefrom, means for applyingsaid unidirectional voltage to said first and second oscillatorsrespectively, phase shifting means for coupling the output of said firstoscillator to the input of said second oscillator to produce outputsfrom said first and second oscillators having the same frequency butbeing displaced in phase with respect to each other an amount inaccordance with a voltage applied to said phase shifting means, meansfor applying the output of said first oscillator and said single phasepower signal as inputs to said first power inverter to produce an outputfrom said first inverter having the frequency and being in phase withthe output of said first oscillator, means for applying said singlephase power signal and the output of said second oscillator to saidsecond power inverter to produce a power output from said secondinverter having the frequency and being in phase with the output of saidsecond oscillator, means in circuit with the output of said invertersfor vectorially combining the outputs therefrom, a reference voltagesource, means in circuit with said last named source and said combiningmeans for comparing7 the voltage of said combining means output withsaid reference voltage to produce a difference voltage therebetween, andmeans for applying said difference voltage to said phase shifter.

28. In combination with a power source for producing an output having avariable voltage and frequency, rectifying means in circuit with saidsource for converting said output to a single substantiallyunidirectional power signal, power switching means comprising first andsecond power inverters, means in circuit with said power source forderiving a unidirectional voltage of a given value therefrom, a firstreference voltage source, means in circuit with said deriving means andsaid first voltage source for comparing said derived voltage with saidreference voltage to produce a first difference voltage therebetween,means for applying said first difference voltage to said deriving meansto produce a regulated unidirectional derived vo-ltage having a chosenvalue, a first magnetic coupled multivibrator comprising a saturabletransformer having a given volt-second characteristic, means forapplying said regulated derived voltage as a supply voltage to saidfirst multivibrator to produce an output from said first multivibratorhaving a frequency which is the function of the magnitude of saidderived voltage and said volt-second characteristic, a second magneticcoupled multivibrator, means for applying said regulated derived voltageas a supply voltage to said second multivibrator, magnetic phaseshifting means for applying the output of said first multivibrator as adriving signal to said second multivibrator to produce an output fromsaid second multivibrator having the frequency of the output of thefirst multivibrator but displaced in phase therefrom, said magneticphase shifting means comprising means having a prescribed volt-secondcharacteristic and which is saturable in a period which is a function ofsaid volt-second characteristic and magnitude of a voltage appliedthereto, means for applying the output of said first multivibrator andsaid unidirectional power signal to said first power inverter to producean output therefrom having the frequency of said multivibrators and inphase with the output of said first multivibrator, means for applyingthe output of said second multivibrator and said unidirectional powersignal to said second power inverter to produce an output therefromhaving the frequency of said multivibrators and displaced in phase withrespect to the output of said first power inverter the same amount asthe phase displacement between said first and second multivibrators,means in circuit with said power inverters to vectorially combine theoutputs therefrom, a second reference voltage source, means in circuitwith said combining means and said second source for comparing thevoltage of the output of said combining means with said second referencevoltage to produce a second difference voltage therebetween, and meansfor applying said second voltage as the voltage for said magnetic phaseshifter.

29. In combination with a power source for producing an output having avariable frequency and voltage, rectifying means in circuit with saidsource for converting said output to a single substantiallyunidirectional power signal, power switching means comprising first andsecond power inverters, means in circuit with said power source forderiving a voltage therefrom, a magnetic amplifer comprising control andgate means, means for applyimg said derived voltage to said gate means,a first reference voltage source, means in circuit with said magneticamplifier and said first source for comparing the voltage output fromsaid magnetic amplifier with said first reference voltage to provide afirst difference voltage therebetween, means for applying said firstdifference voltage to said control means to produce a regulated voltageat the output of said magnetic amplifier, a first magnetic coupledmultivibrator comprising a saturable transformer having a prescribedvolt-second characteristic, means for applying said regulated voltage tosaid first multivibrator to produce an output therefrom having afrequency in accordance with the amplitude of the said regulated voltageand said volt-second characteristics, a second magnetic coupledmultivibrator, magnetic phase shifting means for applying the output ofsaid first multivibrator as a driving signal to said secondmultivibrator to produce an output from said second multivibrator havingthe frequency of said first multivibrator but displaced in phasetherefrom, said magnetic phase shifting means comprising saturable meanshaving a predetermined volt-second characteristic whereby said magneticphase shifting means is saturable in a period which is in accordancewith its volt-second characteristic and a voltage applied thereto, meansfor applying the output of said first multivibrator and saidunidirectional power signal as inputs to said first power inverter toproduce a power output therefrom having the frequency of saidmultivibrators, means for applying the output of said secondmultivibrator and said unidirectional power signal to said second powerinverter to produce an output therefrom having the frequency 0f saidmultivibrators but displaced in phase with respect to the output of saidfirst power inverter the same as the displacement in phase between theoutputs of first and second multivibrators, means for vectoriallycombining the outputs of said power inverters, filter means in circuitwith the output of said combining means for substantially removingtherefrom components having a frequency other than the frequency of saidmultivibrator outputs, a second reference voltage source, means incircuit with said second voltage source and said filter means forcomparing the voltage of the output of said filter means with saidsecond reference voltage to derive a second difference voltagetherebetween, and means for applying said second difference voltage asthe voltage for said magnetic phase shifting means.

30. In the combination defined in claim 29 wherein means are includedfor deriving said second reference voltage from the output of saidcombining means.

3l. ln the combination defined in claim 29 wherein said magnetic phaseshifting means comprises a saturable reactor coupling said first andsecond multivibrators.

32. In the combination defined in claim 29 wherein said magnetic phaseshifting means comprises a magnetic amplier having gate means couplingsaid first and second multivibrators and control means, said seconddierence voltage being applied to said last named contro-l means.

33. ln the combination defined in claim 32 wherein each of said powerinverters comprises an output transformer and wherein the saturabletransformer of said yftrst oscillator comprises a pair or like cores, afirst winding around one of said cores in one polarity, a second windingaround the other of said cores in the opposite polarity, a plurality ofprimary and secondary windings encompassing both of said cores and meansfor initially applying said regulated voltage to said first and seco-ndwindings wlzereby said cores are initially saturated in oppositedirections, there thereby being produced initially from said firstoscillator, a part cycle which is less than 180 electrical degrees.

34. ln the combination defined in claim 33 wherein said magneticamplifier of said magnetic phase shifting means includes first controlmeans to which said second difjerence voltage is applied and secondcontrol means, said second control means being so poled whereby saidmagnetic phase shifting means is initially saturate-d, and wherein eachof said multivibrators comprises a pair of active devices which areconductive during alternate half cycles, said combination furtherincluding means in circuit with a chosen active device of each of saidoscillators respectively for insuring that said chosen active devicesare the first to be rendered conductive to produce outputs from saidfirst and second multivibrators which are initially in phase.

35. In the combination defined in claim 29 wherein each of said powerinverters comprise a pair of inverter elements in bridge arrangement.

36. In the combination defined in claim 35 wherein each of said powerinverter elements comprise first and second silicon controlledrectifers, each of said silicon controlled rectifiers being alternatelygated into conductivity in response to a half cycle of output of thesame polarity from a multivibrator, a first silicon controlled rectifierof one power inverter eletnent and a second silicon controlled rectifierof the other power inverter element being substantially simultaneouslyrendered conductive.

37. In the combination defined in claim 29 wherein said filter meanscomprises a first series combination, tuned' to said frequency, of afirst capacitor and a first saturable inductor in series arrangementwith the output of said combining means and a second parallelcombination, tuned to said frequency, of a second capacitor ana' asecond saturable inductor connected in parallel with the output of saidcombining means, said saturable inductors saturating at chosen currentlevels to detune said resonant combinations,

38. In the combination defined in claim 37 wherein said filter meansfurther includes a saturable output transformer, a portion of which issaid second inductor, and across which the output of said filter meansis developed, said last named transformer saturating at overvoltages.

39. In the combination with a power source for producing an outputhaving a variable voltage and frequency. rectifying means in circuitwith said source for converting said output to a single substantiallyunidirectional power signal, power switching means, means for deriving avoltage from said output, a first magnetic amplifier comprising firstcontrol and frst gate means, means for applying said derived voltage tosaid gate means, a first reference voltage source, means in circuit withsaid first source and said first magnetic amplifier for comparing tlzeoutput of said jrst magnetic an'tplifier with said ft-st referencevoltage to provide a first difference voltage therebetween, means forapplying said rst dierence voltage to said first control means toprovide at the output of said first magnetic amplifier a regulatedderived voltage, a magnetic coupled multivibrator comprising a saturabletransformer having a given volt-second characteristic, means forapplying said regulated derived voltage as a supply voltage to saidmultivibrator to produce a square wave having a frequency which is inaccordance with the amplitude of said derived voltage and saidvolt-second characteristic, means for applying the output of saidmultivibrator and said power signal to said power switching means toproduce a power output having the frequency of said multivibrator, fltermeans in circuit with said power switching means for substantiallyremoving from the output thereof components other than those having thefrequency of said multivibrator, a second reference voltage source,means in circuit with said second source and said filter means forcomparing the voltage of the output of said filter means with saidsecond reference voltage to derive a second difference voltagetherebetween, a second magnetic amplifier comprising second gate andsecond control means, mteans for applying the output of saidmultivibrator to said second gate means and for applying said seconddifference voltage to said second control means to provide an outputfrom said second magnetic amplifier which is in accordance with saidsecond difference voltage, and means for applying the output of saidsecond magnetic amplifier to said power switching means as an errorvoltage.

40. In the combination defined in claim 39 wherein said lter meansincludes a series connected first saturable indicator and a firstcapacitor in series arrangement with the output of said power switchingmeans and a parallel connected second capacitor and a second saturableinductor connected across the output of said power switching means, saidinductors and capacitors being respectively tuned to the frequency ofsaid multivibrator, said inductors saturating when the current in saidpower switching means output exceeds a predetermined value.

4l. In the combination defined in claim 40 wherein said filter meansfurther includes a saturable output transformer, a portion of which issaid second inductor, and across which the output of said filter meansis developed, said saturable transformer saturating at overvoltages tothereby limit tlze periods and magnitudes of over-voltage transients.

References Cited The following references, cited by the Examiner, are ofrecord in the patented le of this patent Or the original patent.

UNITED STATES PATENTS 2,643,358 6/1953 Murray 321-5 3,136,957 6/1964Putkovich et al. 321-5 X 2,875,351 2/1959 Collins 321-2 3,026,484 3/1962Bennett et al 331-113.1 3,031,629 4/ 1962 Kadri S31-113.1

OTHER REFERENCES AIEE Transactions, A More Stable B-PhaseTransistor-Core Power Inverter, Nov. 1959, pp. 686-691.

JOHN F. COUCH, Primary Examiner W. H. BEHA, JR., Assistant Examiner

